1. Field of the Invention
The present invention relates to methods for producing chalcopyrite compound (e.g., copper indium selenide (CIS), copper indium gallium selenide (CIGS), copper indium sulfide (CIS) or copper indium gallium sulfide (CIGS)) thin films that can be used as light-absorbing layers for thin-film solar cells. More specifically, the present invention relates to methods for producing chalcopyrite compound thin films as light-absorbing layers for thin-film solar cells based on solution processes, such as printing, particularly, multi-stage coating of pastes or inks of precursors having different physical properties. The use of chalcopyrite compound thin films produced by the methods enables the fabrication of thin-film solar cells with improved efficiency at low costs.
2. Description of the Related Art
Solar cells can produce electricity directly from sunlight, which is a clean and safe energy source. For this reason, solar cells have attracted considerable attention as the most promising future candidates for energy production. Various kinds of inorganic and organic semiconductors are applied to the fabrication of solar cells. Representative examples of solar cells that have been commercially successful to date include silicon solar cells using silicon (Si) as a main material, and CIGS thin-film solar cells. Silicon solar cells have the advantage of high photoelectric conversion efficiency but suffer from high fabrication costs. Under such circumstances, thin-film solar cells using compound semiconductors that can be formed into thinner films are of growing interest as potential replacements for silicon solar cells.
As representative examples, mention may be made of thin-film solar cells using, as light-absorbing layers, chalcopyrite thin films including Group IB, IIIA and VIA elements known as CIS or CIGS. In such a kind of solar cell, a light-absorbing thin-film layer having the composition of Cu(In,Ga)(S,Se)2 and a buffer thin-film layer composed of CdS or another n-type compound semiconductor are considered the most crucial elements. Particularly, the CIS or CIGS light-absorbing layer is the most important factor determining the performance of the solar cell.
The CIS or CIGS light-absorbing layer is typically produced by co-evaporation or sputtering of metal elements. Specifically, the CIS or CIGS thin film may be deposited by co-evaporation of several components, usually three components, through several stages. Alternatively, the CIS or CIGS thin film may be produced by sputtering Cu, In and Ga metal targets to deposit Cu, In and Ga, followed by selenization. However, all such processes are carried out under vacuum conditions that require expensive vacuum systems. The use of the vacuum systems is responsible for the loss of vast amounts of the costly raw materials, such as indium or gallium, and makes the production of the thin film on a large are at high processing speed difficult.
On the other hand, methods for producing CIGS thin films by inexpensive chemical processes instead of using vacuum systems are known as substitutes for vacuum deposition processes. Particularly, methods for producing CIGS thin films by printing are known as the most promising in terms of processing speed, processing cost and large-area production. Methods for producing CIGS thin films by printing can be broadly divided into two groups of methods. The first group uses an ink or paste composed of precursors. The second group uses an ink or paste prepared by dispersion CIG or CIGS nanoparticles.
As an example of the first group of methods using precursors, Mitzi et al. reported a method for the production of a CIGS thin film which includes dissolving binary compounds, such as Cu2S, In2Se3 and Ga2Se, in hydrazine as a solvent to prepare an ink of the precursors, depositing the ink on a conductive substrate, and thermally treating the deposited substrate in a nitrogen atmosphere [Mitzi et al. Advanced Materials, 2008, 20, 3657-3662]. As another example, Korean Unexamined Patent Publication No. 10-2009-0092471 discloses a method for producing a CIGS thin film which includes dissolving Cu, In and Ga nitrates and SeCl4 in an alcohol as a solvent, mixing the solution with an organic binder, etc. to prepare a paste, depositing the paste on a conductive substrate, and thermally treating the deposited substrate in a H2/Ar atmosphere.
As an example of the second group of methods using nanoparticles, U.S. Patent Publication No. 2006/0062902 discloses a method for producing a CIGS thin film which includes synthesizing and dispersing CIGS nanoparticles, depositing the dispersion on a conductive substrate, and thermally treating the deposited substrate. Another method is known in which CuInGa oxide nanoparticles are synthesized, dispersed and deposited on a conductive substrate, and thermally treated in a H2Se atmosphere to produce a CIGS thin film [Kapur et al. Thin Solid Films 2003, 431-432, 53-57].
Several problems of the conventional methods for the production of CIGS thin films based on printing are causes of low efficiency. The most important one of the problems is associated with the degree of compaction of thin films. Generally, thin films produced by vacuum deposition processes have very densely packed structures substantially free of pores, whereas thin films produced by solution processes have many pores formed during evaporation of solvents or organic additives by thermal treatment. Constituent materials of buffer layers (e.g., CdS), window layers (e.g., ZnO) or metal electrodes (e.g. Al) penetrate thin films through pores in the fabrication of solar cell devices to form shunt paths, which cause low voltage generation and eventually result in deterioration of solar cell efficiency. In view of this, there is a need to develop methods for producing thin films that are based on solution processing while minimizing the formation of pores, thus being suitable for the fabrication of high-efficiency thin-film solar cells.
There is also a need to develop methods for producing CIGS thin films based on solution processing by which the concentration distributions of the constituent elements in the thin films can be easily controlled. For example, a method for producing a thin film based on solution processing designed such that the concentrations of Ga in the front and back portions of the thin film are higher than the concentration of Ga in the intermediate portion of the thin film, needs to be developed. This concentration difference increases the local band-gap of a predetermined area in the thin film to prevent the frequent occurrence of electron-hole recombination at the interfaces, contributing to an increase in overall efficiency as well as Voc. Therefore, the method would be very important in the realization of high-efficiency chalcopyrite compound thin-film solar cells at low costs.